The Educational Institute of Scotland's leading thinker, George MacBride, has launched a stinging attack on HM Inspectorate.
Mr MacBride, the union's education convener, suggests inspectors are in danger of becoming "acquiescent servants of Government ministers and unaccountable decision makers". His rebuke comes amid signs of growing political scrutiny of HMI.
Writing in the latest edition of the Scottish Educational Journal, the official organ of the EIS, Mr MacBride accuses the inspectorate of using questionable evidence in national reports which are not backed by research.
He says it is "odd that even research commissioned by the Scottish Office Education and Industry Department, such as that into streaming and setting and that into the implementation of the 5-14 programme, merits almost no mention in HMI papers.
"Some might consider that this implies a certain arrogance, in that all evidence save that gathered personally by HMI is not considered worthy of consideration; others might be tempted to see it as a sign of insecurity."
Douglas Osler, the senior chief inspector of schools, has made a virtue out of what he regards as the inspectorate's unique vantage point. He says this allows for more representative judgments than can flow from "small-scale research".
He has, for example, frequently cited the comprehensive nature of the 1996 report on Standards and Quality in Scottish Schools, which was based on inspection evidence from 9,000 primary and 12,000 secondary classes.
Mr MacBride, on the other hand, argues that "it has become commonplace for such documents as Achievement for All and Achieving Success in S1S2 to make assertions about current practice unsustained by stated evidence derived from observation; reflection is replaced by pronouncement."
He finds it ironical that a body which observes practice in Scottish education has failed to notice what he sees as the unreliability of the national testing regime and "the contempt in which national tests are held by many of the teachers who use them".
Mr MacBride also takes issue with the tone of HMI pronouncements. Unlike the best practice of teachers, these are seen as highlighting weaknesses rather than strengths.
He accuses HMI of "hectoring others to remedy these perceived 'faults' for which they themselves take no responsibility".
The EIS wants the inspectorate to abandon centralism in favour of "subsidiarity", which would give schools and education authorities more say in how they implement national policies.
Despite the EIS strictures, Mr Osler has not acquired the anti-teacher reputation of his English counterpart Chris Woodhead, the head of the Office for Standards in Education.
Indeed, at the recent spring conference of the Headteachers' Association of Scotland Mr Osler distanced himself from accusations of talking down to the profession. But he said he still has a duty as adviser to the Secretary of State to highlight areas requiring improvement.
He paid tribute to the 80 per cent of heads who, he said, were performing well in one of the most difficult of management roles. "I have refused to become involved in negative rhetoric which causes unnecessary alarm and despondency among parents, and which fails to acknowledge the successes in the system," Mr Osler said.
He confessed to being "irritated" at some post-mortem comments following the publication of exam results. These imply that when results have improved, it is because the exams have become easier, not because standards have risen or teaching has become more effective.
The EIS attack follows recent demands in the Commons by Michael Connarty, the Labour MP for Falkirk East, for the education minister to "rein in" Mr Osler and his departmental officials. Mr Connarty claimed at least 55 Scottish Office directives had been issued since the election last May.